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Google has removed four malicious browser extensions with a combined total of 500,000 downloads from the Chrome Web Store.

Security firm ICEBRG discovered the malicious extensions after it picked up an unusual spike in outgoing network traffic. The first extension identified was called HTTP Request Header. After further investigation, it discovered three more: Nyoogle, Stickies and Lite Bookmarks.

In a blog post, ICEBRG said that although the extensions were probably used to commit click fraud (imitating the process of a user clicking an ad in their browser) or manipulate search engine results, they could be used to create a botnet with the potential to access business networks and user information.

"Removal of the malicious extension from the Chrome Web Store may not remove it from impacted hosts," ICEBRG added. "Additionally, the use of third-party Chrome extension repositories may still allow the installation of the extensions."

It's possible that the number of downloads could have been inflated through use of bots to make the extensions seem legitimate and trick more people into installing the malware.

Verifying Browser Extensions

Each browser developer takes a different approach to verifying the quality and safety of add-ons in their stores.

Before publication on the Google Chrome Web Store, extensions are subjected to a process called Enhanced Item Evaluation -- a series of automated checks that examine its code and behavior once installed to identify malware. Once the validation is complete, the app is published -- usually within an hour.

The process normally works well, but sometimes extensions slip through the net. For example, in October last year, 37,000 people downloaded a fake version of Adblock Plus that was almost impossible to differentiate from the real thing.

Microsoft launched an extension store for Edge in 2016, and tests each submission individually before it's published -- a process than can take 72 hours.

Mozilla takes a more liberal approach. All Firefox add-ons must comply with a set of policies and practices that varies depending on circumstances. Extensions listed on addons.mozilla.org may be subject to automatic and manual review and testing, during which they won't appear in search results, but will still be accessible if you have a link to their listing pages.

Unlisted add-ons aren't subjected to quite such strict standards. They must still be uploaded to addons.mozilla.org, but have to be distributed elsewhere.